I'm seeing this in a 5.5-stable 'make build'. Everything from CVS sync up to this point works fine, and the kernel already built and installed works great. This was Release but I wanted to catch the recent SSL fixes, but this is a no-X firewall so maybe I don't even need -stable and can go back to Release.

I will guess that, if you have done nothing operationally incorrect, that you are the victim of a recent problem with a transition of CVSync versions, which is the tool used worldwide for OpenBSD's AnonCVS mirrors. This will have affected -stable tags (OPENBSD_5_5) as well as -current (HEAD).

I recommend you contact the maintainer of your mirror directly (the maintainer's Email addresses is listed in http://www.openbsd.org/anoncvs.html#CVSROOT and ask if it is now safe to run cvs(1) update to obtain a correct working directory in /usr/src.